Receptor G protein interaction. Mathematical modeling of hormone-receptor- G protein interaction predicts that receptors can be coupled in the absence of agonists and that this spontaneous interaction can be revealed using antagonists with negative efficacy. Using this type of antagonist, we have determined that opioid receptors and G proteins undergo spontaneous interactions in isolated membranes but not in intact cells. This suggests that factors present in the cytosolic environment play a fundamental role in suppressing ligand-independent receptor activity. We extended the model to describe simultaneous equilibria between three proteins (receptor, alpha-subunit and beta-subunit of G protein) and two ligands (hormone and GTP nucleotides). This model predicts that guanine nucleotides and beta-subunits act synergistically to suppress receptor-generated noise and maximize the dynamic range of hormonal-induced stimuli. G Proteins and mitogenic responses. It is believed that increased phosphoinositide hydrolysis and the consequent calcium mobilization are major signalling pathways by which many peptides promote mitogenic responses in cultured cells. We have studied a human neuroblastoma cell line in which thrombin is a potent mitogen. Thrombin receptor-induced initiation of DNA synthesis is mediated by two distinct signalling pathways. One involves PI turnover, and is mediated by a G protein which is not a substrate of pertussis toxin (Gp). The other is independent of phospholipase C and calcium and requires pertussis toxin-sensitive G proteins (Gi2 and Gi3). The activation of Gi usually results in decreased levels of cAMP. In contrast, thrombin increased cAMP levels, by a mechanism dependent on calcium mobilization, since it was prevented by pretreatment of cells with phorbol esters but unaffected by pertussis toxin. Thus, thrombin receptor mediated control of DNA synthesis via Gi does not involve cAMP, but still unknown effector systems.